Pneumatic rock drill with selective drill steel rotation



FUEHRER DRILL STEEL ROTATION 3 Sheets-Sheet 1 Filed Sept. 9, 1953 5 mmex u Hm e m N 0 mm 7 Q @N H mm ow mm 3 Q E w. w N i i 42/ m 0Q mm hm. \QG mo 5 kwm 6 mo J 0 B P5. $9 \xmwmmxm w Q 8 4 wv /mm M mm @m .I l. Em iN Q. mm mm h s 6 cm m W :Kfi 274/ V m 7 Dec. 13, 1955 G. H. FUEHRER2,726,640

PNEUMATIC ROCK D L WITH SELECTIVE DRILL STE ROTATION 3 Sheets-Sheet 2Filed Sept. 9, 1955 IN V EN TOR.

650 965 b. FUE'H/QEIQ, W

Dec. 13, 1955 cs. H. FUEHRER 2,726,640

PNEUMATIC ROCK DRILL WITH SELECTIVE DRILL STEEL ROTATION 3 SheetS-Sheet3 Filed Sept. 9, 1955 INVEN TOR.

/? QA/EV 660865 H FUf/VQEQ,

United States l atent PNEUMATIC ROCK DRILL WITH SELECTIVE DRILL STEELROTATION George H. Fuehrer, Downey, Calif., assignor to Thor Power ToolCompany, Aurora, IlL, a corporation of Delaware Application September 9,1953, Serial No. 379,207

7 Claims. (Cl. 121-7) Steel Rotation, now Patent No. 2,678,636, issuedMay 18,

1954, I have disclosed a relatively light air hammer for use indrilling, chipping and the like, and which incorporates means to preventsteel rotation when it is desired to form a starting hole or when theapparatus is to be used as a chipping hammer. In the air hammer referredto, manually operated means were provided to efiect shifting of therotation pawls longitudinally of the drill and into engagement with acam surface adapted to pivot them away from the associated ratchet ringand thus prevent steel rotation. The present invention also involves thebroad principle of longitudinal pawl shifting for the purpose ofpreventing drill steel rotation while permitting the hammer action tocontinue. However, the present invention is specific to the embodimentof the principle in a relatively large drilling apparatus such as astoper or sinker employed in mining and tunneling operations.

It is evident that the problem encountered in connection with relativelylarge drilling apparatus, such as is used for mining and tunneling, aresomewhat different than those which must be overcome in connection withsmall air hammers such as the one described in the copending applicationreferred to above. For example, the relatively large size of the drillmakes it impractical and inconvenient to effect pawl shifting by manualaction alone. Furthermore, the camming and spring return means for thepawls must be associated with the valve guide, rifle bar and otherelements of the drill in such a way that the rotation release devicewill not interfere with normal drill operation.

In view of the above factors characteristic of relatively largepneumatically operated drilling apparatus, it is an object of thepresent invention to provide novel means for pivotally mounting therotation pawls in the rifle bar flange of a rock drill, the means beingsuch that longitudinal shifting of the pawls is permitted to effecttheir pivotal movement out of operative engagement with at ratchetring.

An additional object is to provied cam and spring return means forassociating the longitudinally shiftable rotation pawls and thestationary valve guide of the rock drill.

A further object is to provide pneumatically operated piston means forshifting the pawls under the control of a manually operable .valve, thelatter being mounted on the main throttle valve for the rock drill butoperating independently thereof.

These and other objects and advantages of the invention will be morefully set forth in the following specification and claims, considered inconnection with the attached drawings to which they relate.

2,726,040 Patented Dec. 13, 1955 In the drawings: a g

Figure 1 is a longitudinal central section of the backhead and adjacentportions of a rock drill of the side-feed stoper type; I

Figure 2 is a vertical section along line 2-2 of Figure 1, with portionsbroken away to illustrate the air passages connecting the control valveand the pawl-shifting piston;

Figure 3 is a fragmentary sectional view along line 33 of Figure 2 andshowing the control valve in its vent position, at which the rotationpawls are engaged with the associated ratchet ring;

Figure 4 is a sectional view along line 44 of Figure 3, and illustratingthe mounting of the rotation control valve on the main throttle valve ofthe drill;

Figure 5 is a sectional view of the rotation control valve,corresponding to Figure 3 but illustrating the valve as shifted to itsrotation-preventing position;

Figure 6 is a fragmentary longitudinal section illustrating the rotationpawls as shifted forwardly from the Figure 1 position to therotation-release position;

Figure 7 is a transverse section of the drilling apparatus, taken alongline 77 of Figure l and showing the pawls in the operative positionsassumed when the parts are in the positions shown in Figures 1 and 3;and

Figure 8 is a view corresponding to Figure 7 but show ing the pawlsafter pivoting to the disengaged positions assumed when the parts are inthe positions shown in Figures 5 and 6.

Referring to the drawings, and particularly to Figure l, the inventionis illustrated as embodied in a stoper-type rock drill of the offsettelescopic feed leg type. The offset feed leg, indicated fragmentarilyat 10, is associated through a bracket arm 11 with the backhead 12 whichcontains the valve elements of the drill. Backhead 12 is, in turn,associated through suitable side bolts 13 (Figures 2, 7 and 8) with themain drill cylinder 14 containing conventional piston hammer, chuck anddrill steel elements. The piston hammer, indicated at 15, is adapted topercussively actuate the drill steel which is mounted in the chuck, bothof these elements being unshown.

The rear end portion of main cylinder 14 is provided with an internalshoulder 16 to seat in stacked abutting relationship, starting from theleft in Figure 1, a forward valve chest 17, a rear valve chest 18, avalve guide 19, and a ratchet ring 20, the latter being also seated onan internal shoulder 22 formed at the forward portion of backhead 12.All of the elements 17-20 are locked I ratchet ring 20 relative tocylinder 14 and backhead 12 is efiectively prevented not only by theside bolts but by a key pin 23 disposed in suitable registered groovesin these elements.

It is to be understood that the valve chests 17 and 18 form part of aconventional distributing valve embodying a longitudinally reciprocablevalve element 24 which is slidable on a forwardly extending cylindricalstem portion 26 of the valve guide 19. The distributing valve operatesto alternately direct air through suitable air passages and to theforward and reverse sides of the piston hammer 15 in order to efiect itsreciprocation for the purpose of imparting both percussion and rotationto the drill steel. The flow of air to the distributing valve is from asuitable inlet air hose, not shown, and through the main throttle valve27 which is mounted transversely of backhead 12 in a rounded bossportion 28 at its under surface. The throttle valve 27 comprises ahollow sleeve or cylinder 29 rotatably mounted in a transverse bore inboss portion 28 and associated through a necked-down end portion 31 witha handle 32. As illustrated in Figure 4, the handle 32 may be removablymounted by use of a nut ,33 which is threaded over the Outer and Of P tand is ada ted to bear nst th base or hub of handle 32, the latter beingseated over a squared section of portion 31 inwardly adjacent itsthreaded end.

Referring again to Figure 1, the piston hammer is associated through itsrifle nut 34 with a rotatable rifle bar 35 which extends through anaxial bore in valve guide stem 26. Rifle bar 35 is provided at its rearend'with a circular flange 36 rotatably seated between the flangeportion 37 of valve guide 19 and a stationary plug 38, the latter beingpress fit into an axial bore in backhead 12. To further stabilize therotational relationship between rifle bar 35, valve guide 19'and'plug38,-'the rifle bar is provided at its rear end with a cylindricalprotuberance 39 which seats in a corresponding cylindrical recess inplug 38. It will be noted that the rifle bar 35 and plug 38 are axiallybored to receive the conventional water tube 41 adapted to supplycleansing water to the bit end of the drill steel; The flow'of waterthrough tube 41 is under the control of a pneumatically operated watervalve 42 of the type described and'claimed in my co-pending applicationSerial No. 355,679 filed May 18, 1953, for an Automatic Water Valve forRock Drills.

According to the present invention, a plurality of rotation pawls 46 aremounted in'rifle bar flange 36 for movement longitudinally and forwardlyof the drill and into engagement with a cam surface provided on flange37 of valve guide 19. As best shown in Figures 7 and 8, the rifle barflange 36 is formed with four peripherally spaced notches 47 each havinga radial edge 48 and a somewhat longer edge 49 extending along a chordof the circle formed by the flange periphery. The notches are adapted toreceive rectangular lug plates or portions 51 of pawls 46 and to permittheir pivotal movement about axes extending longitudinally of drillcylinder 14. To provide for the pivotal movement of the lug plates asindicated, and also to permit shifting of the p'awls'longitudinally ofthe drill, the inner edge portion of each lug plate 51 is integral atits center with a cylindrical journal portion 52 adapted to be'slidably'received-in a longitudinally extending'cylindri'cal bore o'rrecess 53formed in the corner of each notch 47. In orderto bias the lugplates 51 radially outwardly into "engagement with the internal teeth 55of ratchet ring 20, and which are'outwardly adjacent the periphery ofrifle bar flange 36, a plurality of resilient plunger mechanisms 56 aremounted one for each pawl in cylindrical recesses 57 extendingperpendicularly of the respective chordal notch edges 49. The plungermechanisms 56, which are respectively mounted in two transverse planes,each comprise a metallic plunger cylinder 58 slidably mounted in itsassociated recess 57 and enclosing a helicalcompres'sion spring 59,there being a bore 61 communicating With'the bottom of each recess 57 topermit exhausting of air in the recess when plunger 58 is shiftedinwardly;

In the normal operation of the drill with both rotation and hammeringaction, the plunger mechanisms 56 are operative to pivot pawl lugplates-51-outwardly into engagement with the ratchet-teeth '55 to effecta ratchet action between the ratchet ring and rifle flange 36; To effectcessation of the ratchet action, and consequent stoppage of drill steelrotation, the pawls 46 are shifted forwardly through notches 47 andrecesses 53 toprovide a camming action as will next be described.

It will be observed upon reference to Figure 1 that the rear surface ofvalve guide flange 37 is capped to form an annular recess 63 between theperipheral flange portion and the valve guide stem 26. Recess 63 isprovided longitudinally opposite the -'rim' Qfriflebar flange 36 with afrustoconical cam wall 64 converging toward the forward or front part ofthe drill. A radially extending wall 66 is provided at the cup bottomtoform"a "seatfor a flat wire helical compression spring 67 which extendsbetween wall 66 and the forward edges 68 of pawl lug plates 51. Thecompression spring 67 operates to bias the pawls 46 rearwardly to theposition shown in Figure 1, and at which the plunger mechanisms 56 areoperative to pivot the pawls outwardly into engagement with the ratchetring teeth 55. However, upon shifting of pawls 46 forwardly against thebias of spring 67, the outer longitudinal corners 70 of lug plates 51ride on cam surface 64 to efiect inward pivoting of the pawls to thedisengaged position illustrated in'Figu'res 6 and 8. The ratchetmechanism is thus maintained in free-wheeling or inoperative positionuntil the pawls are permitted to return to their rearward-shiftedpositions under the'bias of spring 67.

In accordance with the present invention, the means for shifting thepawls 46 forwardly to provide with cam surface 64 a cam action effectingretraction of pawl portions 51 out of engagement with ratchet teeth 55,comprise a cup-shaped piston 72 which is slidably mounted over plug 38for movement longitudinally of the drill. Piston 72 is shaped anddisposed with its rim adjacent the rear edges 74 of pawl lug plates 51,and with its base 75 necked-down toslidably engage a relatively smalldiameter portion 76 of plug 38. It'will be observed (Figures l and 6)thatthe forward travel of piston 72 is limited by the radially extendingplug wall 77 which separates small diameter portion 76 from themai'nbody 78 of the plug, and the rearward travel by a radial backheadwall 79 which forms the bottom of a'recess in backhead 12 for piston 72.The radial backhead wall 79 forms the forward surface of an internalshoulder or neck 80 in backhead 12 and which receives the relativelysmall diameter portion 76 of plug 38 in press-fit relationship aspreviously indicated.

Proceeding next with the description of the air passage and valve meansfor shifting piston 72 forwardly'to the rotation-release position shownin' Figure 6, it will be observed upon reference to Figures 2 and 4 thata relatively small rotation control or'release valve 81 is mountedcircumferentially of the necked-down end 31 of throttle valve sleeve 29and inwardly of the throttle valve handle 32. The rotation release valve81' comprises a sleeve'82 mounted on a cylindrical section of throttlevalve portion 31 and adapted to rotate relative to it, there being aboss or flange 83 provided on' sleeve 82 between the main body ofthrottlevalve sleeve 29 and the neck-down end portion of boss 28. Aradially extending handle 84 is mounted over sleeve 82 between boss 28and throttle valve handle 32 and is provided with a screw 86 projectingthrough an aperture in sleeve 82 and into an angular groove 87 inthrottle valve extension 31, the arrangement being such that turning ofhandle 84 will rotate sleeve .82 independently of the throttle valve. Aspring-pressed detent mechanism 88, mounted in backhead boss 28, isadapted to cooperate with handle 84 to maintain the valve 81 ina's'el'ected controlling position except during intentional shifting ofthe valve by the operator. With the described construction the op eratormay independently control the throttle valve 27 and the rotation releasevalve 81 by manipulating valve handles 32 and 84 which are disposedclosely adjacent each other forease of access.

Referring again to Figure 4 of the drawings, it is to be assumed thatthe valve chamber 90 defined by throttle valve sleeve 29 is supplied:with compressed'air from a suitable air hose, notshown,'for conductionto the distributing valveas previously indicated. According to thepresent invention the compressed air in chamber 90 is alsoemployed,"under the control, of valve 81 to shift rotation-releasepiston 72 forwardly and effect 'clisengagementof pawls 46 from r mnajr iaa Eorthis' purpose, a bore 91 is formed in throttleyalve sleeve 29between chamber 90 and an annulangroove 9 2at the meeting'faces ofsleeve 2 9ja'nd "flang'e 83. In addition, the flange 83 is provided witha passage 93 leading from groove 92 to the cylindricall'wall offthebackhead bore into which throttle valve sleeve 29 is inserted. When thesleeve 82 is rotated by handle 84 to the rotationrelease positionillustrated in Figure 5, passage 93 registers with a passage 94 (Figures2, 3 and leading upwardly through backhead 12 to a radially inwardlyextending passage 96. Passage 96, in turn, communicates with an annulargroove 97 formed in the rear surface of piston 72 as shown in Figures 1,2 and 6. It may be seen that when the valve is in the Figure 5 position,air pressure is transmitted to groove 97 and will operate to shiftpiston 72 forwardly until piston base 75 engages wall 77 of plug 38.

As the piston is thus shifted forwardly, the air between the wall 77 andpiston base 75 is vented to the atmosphere by means of communicatingpassages 98 and 99 in plug portion 76, the latter passage leading manannular groove 101 in portion 76 which registers with a passage 102 inbackhead 12. Passage 102 communicates with a laterally extending passage103 (Figures 1 and 2) which leads to the atmosphere and completes a ventpath for the air chamber between piston base 75 and plug wall 77.

It is to be understood that air pressure admitted to annular pistongroove 97 and thus to the space between piston base 75 and backhead wall79 is sufficient to press the piston and pawls 46 forwardly against thebias of spring 67 and also against the air pressure admitted fromthrottle valve 27 to the chamber 104 surrounding the pawl and ratchetmechanism and the piston. The chamber 104 includes in its forwardportion the recess 63 in valve guide flange 37 and in its rear portion.a recess formed in backhead 12 outwardly of the cylindrical body ofpiston 72. Chamber 104 forms part of the conduit means through which airpasses from throttle valve 27 on its way to the distributing valvemechanism and thus to the piston hammer 15, there being a port 106 inthrottle valve sleeve 29 and a passage 107 in backhead 12 for thepurpose of connecting valve chamber 90 with chamber 104.

In order to eifect recommencement of drill steel rotation after thepawls 46 have been shifted forwardly to their rotation-releasepositions, the rotation control valve 81 is turned to the positionillustrated in Figure 3. The flow of air from valve chamber 90 tobackhead passage 94 is thus blocked, since passage 93 in flange 83 thenleads only to a Wall of the bore for sleeve 29. A vent path from thechamber between piston base 75 and backhead wall 79 is, however, createdthrough groove 97 and passages 96 and 94 to a recess or groove 109 inthe outer surface of flange 83. Recess 109 communicates, when therotation control valve 81 is in the Figure 3 position, with upwardly andtransversely extending back head passages 111 and 112 which lead to theatmosphere as indicated in Figure 2. When the flow of air to pistongroove 97 is blocked and the vent path completed as described, thepiston 72 and pawls 46 are shifted rearwardly from their Figure 6positions to their original positions shown in Figure l. The rearwardshifting is efiected by the combined action of compression spring 67 andof the air pressure in chamber 104, with the spring acting on the pawls46 and the air pressure acting on the rim of piston 72.

In the operation of the rock drilling apparatus, let it be assumed thatthe valve 81 is in the position illustrated in Figure 3, so that piston72 is in its rearward shifted position (Figure l) at which the pawl lugplates 51 are in engagement with ratchet teeth 55 as shown in Figure 7.Let it also be assumed that the main throttle valve 27 is turned to asuitable position at which air is admitted through port 106 and passage107 to chamber 104 and thence to the distributing valve to effectreciprocation of valve element 24 for alternate transmittal of air tothe forward and rear sides of piston hammer 15. As previously stated,the piston hammer 15 is provided, as in conventional rock drillingapparatus of the type indicated, with the rifle nut 34 which rides onthe spirally fluted rifle bar 35 and tends to eflect relative rotationbetween the rifle bar and piston hammer during reciprocation of thelatter. During intervals when rifle bar 35 is held against rotation thepiston will rotate, and during intervals when the rifle bar is permittedto move freely the rifle bar will rotate and the piston will not.Accordingly, the pawl and ratchet mechanism is provided to lock riflebar 35 against rotation during each rearward piston stroke, so that thepiston hammer rotates to rotate the associated chuck and drill steel,but to permit rifle bar rotation during each forward stroke to effecttemporary cessation of piston and drill steel rotation. The pistonhammer, chuck and drill steel are thus rotated during each rearwardpiston hammer stroke and in a single direction as is desired. It will beobserved that when the parts are in the assumed positions the pawl lugplates 51 are in engagement with ratchet teeth 55, under the resilientbias of plunger mechanisms 56. The necessary ratchet action is thuseffected between the stationary ratchet ring 20 and the rotatable riflebar 35 to effect drill steel rotation in the conventional mannerindicated above.

Should it be desired to effect cessation of drill steel rotation, forexample when it is necessary to form a starting or collar hole or whenthe drill steel is caught in a seam, rotation control valve handle 84 isrotated until the valve is in the position illustrated in Figure 5. Anair 'path is then completed from throttle valve chamber 90 (Figure 4)through passage 91 in valve sleeve 29, passage 93 in flange 83, passages94 and 96 in backhead 12 and to the annular groove 97 in piston 72.Piston 72 is then shifted forwardly against the air pressure in chamber104, which operates to shift the pawls 46 forwardly against the bias ofspring 67 and to the rotation-release positions shown in Figure 6. Asthe pawls are thus shifted, the outer corners 70 of pawl plates 51 ridealong the frustoconical cam wall 64 of valve guide flange 37 and aredrawn inwardly against the bias of plunger mech-' anisms 56 to thepositions shown in Figure 8, at which they are in engagement with thechordal notch walls 49. The pawl lug plates 51 are then out ofengagement with ratchet teeth 55 to effect free-wheeling between therifle bar flange 36 and ratchet ring 20. The rifle bar 35 is thenrotated back and forth during reciprocation of the piston hammer toslide the pawl edges on the cam wall 64, there being no rotation of thepiston hammer, chuck and drill steel since the rifle bar is not lockedagainst rotation in either direction.

To again start the drill steel rotation, the valve 81 is shifted to itsFigure 3 position, at which a vent path is completed from groove 97 andthrough backhead passages 96 and 94 to recess 109 in the valve flange 83and thus to passages 111 and 112 leading to the atmosphere. The spring67 and the air pressure in chamber 104 are then operative to shift pawls46 and piston 72 rearwardly until piston base engages backhead wall 79as shown in Figure 1. During rearward shifting of the pawls 46, theplunger mechanisms 56 act on the respective pawl lug plates 51 to pivotthem outwardly into engagement with ratchet teeth 55. The pawls 46 thenoperate to prevent rotation of the rifle bar 35 in a clockwise directionas viewed in Figures 7 and 8, so that the piston hammer, chuck and drillsteel will instead rotate as desired.

Referring to Figure 1, it is pointed out that the pawl return spring 67is completely expanded and at free length when pawls 46 are in theirrearward-shifted positions. The piston 72 is also in itsrearward-shifted position due to the action of the air pressure inchamber 104,. and the arrangement is such that clearance gaps are thenformed between pawl ends 74 and the piston rim. In this. manner, the airpressure in chamber 104 is utilized to completely disengage piston 72from the pawls and pre-- vent any interference with the normal pivotingoperation of the latter.

I i Pointed u tha th t r l v e 27 y b turned to any controlling positionindependently of the rotation control valve 81 and without affecting thelatter, since the annular groove 92 (Figure 4) is engaged with valvepassage 93 during all positions of the rotation-release valve.Completely independent control of the hammer action and rotation actionis thereby achieved, yet the valve handles 32 and 84 are sufficientlyclose to permit an operator to manipulate them without shifting his gripon the drill.

While the particular apparatus herein shown and described in detail isfully capable of attaining the objects and providing the advantageshereinbefore stated, it is to be understood that it is merelyillustrative of the pres ently preferred embodiments of the inventionand that no limitations are intended to the details of construction ordesign herein shown other than as defined in the appended claims.

I claim:

1. A pneumatic rock drilling apparatus, comprising an elongated casingadapted to mount a drill steel, a piston hammer mounted in said casingfor reciprocation toward and away from said drill steel to percussivelyactuate the same, a valve guide mounted longitudinally in said casingand adapted to slidably mount the distributing valve of the apparatus, aflanged rifle bar journalled longitudinally in said valve guide andadapted to cooperate with a nut on said piston to effect relativerotation between said piston and rifle bar during reciprocation of saidpiston, an internally toothed ratchet ring mounted peripherally of theflange of said rifle bar and locked against rotation relative to saidcasing, a pawl mounted on said rifle bar flange for pivotal movementabout, an axis extending longitudinally of said casing and for shiftingalong said axis, a flange provided on said valve guide adjacent saidrifle bar flange, said valve guide flange being recessed to form a camsurface adapted when engaged by said pawl to pivot the same out ofengagement with the teeth of said ratchet ring, and means to shift saidpawl along said axis and into engagement with said cam surface.

2. The invention as claimed in claim 1, wherein a compression spring isseated between the bottom of said recess and said pawl to bias thelatter toward its initial longitudinal position.

3. The invention as claimed in claim 2, wherein a compression spring ismounted on said rifle bar flange and adapted to bias said pawl intoengagement with said ratchet ring teethexcept when said pawl is held indisengaged position.

4. In a pneumatic drilling apparatus of a type employinga rotatablerifle bar to effect rotation of the piston hammer during itsreciprocation, means to define a chamber adapted to contain air underpressure, a rifle bar flange mounted in said chamber and rotatable withsaid rifle bar, a ratchet ring fixedly mounted adjacent said rifle barflange, a plurality of pawls mounted on said rifle bar flange forpivotal movement into engagement with said :8 ratchet ring and formovement longitudinally of the drilling apparatus, a cam surfaceprovided longitudinally adjacent said pawls and adapted when engagedthereby to eflect pivoting thereof out of engagement with said ratchetring, spring means to bias said pawls toward a position out ofengagement with said cam surface, and pneumatically operated pistonmeans to shift said pawls longitudinally into engagement with said camsurface; said piston means being normally held in inoperative positionby the air pressure in said chamber, and being constructed and adaptedto overcome both said air pressure and the bias of said spring meanswhen operated to longitudinally shift said pawls.

5. The invention as claimed in claim 4, wherein said chamber definingmeans includes a flange on the valve guide for said apparatus, saidflange being also constructed to seat said spring means and to form saidcam surface.

6. The invention as claimed in claim 4, wherein said spring means are atfree length when said piston means are in said inoperative position, andwherein clearance is then provided between said piston means and pawlsto prevent interference with the normal operation of the latter.

7. A pneumatic rock drilling apparatus, comprising an elongated casingadapted to mount a drill steel, a piston hammer mounted in said casingfor reciprocation toward and away from said drill steel to percussively'actuate the same, a valve guide mounted longitudinally in said casingand adapted to slidably mount the distributing valve of the apparatus, aflanged rifle bar journalled longitudinally in said valve guide andadapted to cooperate with said piston to eflect relative rotationbetween said piston and rifle bar during reciprocation of said piston,an internally toothed ratchet ring mounted peripherally of the flange ofsaid, rifle bar and locked against rotation relative to said casing, apawl mounted on said rifle bar flange for pivotal movement about an axisextending longitudinally of said casing and for shifting along saidaxis, a flange provided onsaid valve guide adjacent said rifle barflange, said valve guide flange being shaped to form a cam surfaceadapted when engaged by said pawl to pivot the same out of engagementwith the teeth of said ratchet ring, a compression spring seated on saidvalve guide flange and adapted to engage one end of said pawl and biassaid pawl toward an initial longitudinal position, a piston mounted insaid casing and slidable to contact the other end of said pawl to shiftsaid pawl into engagement with said cam surface, and avalve-controlled.pneumatic system selectively operable to actuate saidpiston.

References Cited in thefileof this patent UNITED STATES PATENTS1,905,474 Lear Apr. 25, 1933 2,001,729 Huffman May 21, 1935 2,251,269Curtis Aug. 5, 1941 2,678,636 Fuehrer May 18, 1954

